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Publication numberUS6124761 A
Publication typeGrant
Application numberUS 09/163,885
Publication dateSep 26, 2000
Filing dateSep 30, 1998
Priority dateSep 30, 1998
Fee statusPaid
Publication number09163885, 163885, US 6124761 A, US 6124761A, US-A-6124761, US6124761 A, US6124761A
InventorsTrevor Robinson, Pramote Piriyapoksombu
Original AssigneeRobinson; Trevor, Piriyapoksombu; Pramote
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Variable gain amplifier with gain linear with control voltage
US 6124761 A
Abstract
A variable gain amplifier (VGA) having a control voltage source that provides high gain-to-control voltage linearity over at least an 80 dB gain range. Further, the gain curve for the VGA is essentially independent of temperature. In the preferred embodiment, the VGA includes a two-stage bipolar differential amplifier. Each stage is a transconductor followed by current steering. The first stage amplifier is coupled to an exponentially varying current source to change the transconductance of the stage. The second stage amplifier is coupled to an fixed current source to maintain a fixed transconductance for the stage. To obtain exponential current steering, the control signal for the current steering circuitry is pre-distorted by the following equation:
I/(1+exp(-f(VCTRL /VT))=I*A*exp(VCTRL /VREF),
where A is a scaling factor, VT =kT/q, and T is temperature in Kelvin. The invention includes a fast, inexpensive control voltage source that provides such a signal.
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Claims(4)
What is claimed is:
1. A variable gain amplifier, including:
at least one differential amplifier stage having an amplifier section and a current steering section exhibiting a hyperbolic tangent characteristic;
a voltage source, coupled to the current steering section of at least one differential amplifier stage, for generating a first control signal output proportional to IEXP, and a second control signal output proportional to ICONST -IEXP, with ICONST being a constant current, and IEXP being exponentially related to an input control voltage VCTRL and independent of temperature, where the first and second control signal outputs control correct steering for the current steering section so that the current steering section has an exponential characteristic, such that the current steering section having said exponential characteristic provides high gain-to-control voltage linearity over a wide dB range.
2. The variable gain amplifier of claim 1, wherein the voltage source includes:
(a) a voltage-to-current converter for converting the input control voltage VCTRL to an output current I1 ;
(b) a current multiplier/divider, coupled to the voltage-to-current converter, for generating an output IOUT equal to I1 *I2 /I3, where I2 is generated by a proportional-to-absolute-temperature current source, and I3 is generated by a constant current source;
(c) an exponential current generator, coupled to the current multiplier/divider, for generating IEXP as an exponential function of IOUT and a constant current I4, and independent of temperature; and
(d) an output circuit, coupled to the exponential current generator, for generating the first control signal output proportional to IEXP and the second control signal output proportional to ICONST -IEXP.
3. The variable gain amplifier of claim 2, wherein the output circuit includes:
(a) a current replicator, coupled to the exponential current generator, for generating two essentially identical output currents equal to IEXP ;
(b) a current subtractor, coupled to one output of the current replicator, for subtracting IEXP from ICONST ;
(c) a first current conversion circuit, coupled to the current replicator, for generating the first control signal output proportional to IEXP as a first voltage; and
(d) a second current conversion circuit, coupled to the current subtractor, for generating the second control signal output proportional to ICONST -IEXP as a second voltage.
4. A method of varying gain in an amplifier circuit, including the steps of:
applying a differential input signal to a differential amplifier having an amplifier section and a current steering section exhibiting a hyperbolic tangent characteristic;
generating a first control signal output proportional to IEXP, and a second control signal output proportional to ICONST -IEXP, with ICONST being a constant current, and IEXP being exponentially related to an input control voltage VCTRL and independent of temperature; and
exponentially steering the output signal current from the differential amplifier either to a voltage source or to a corresponding output in response to the first and second control signal outputs, where said exponentially steering provides high gain-to-control voltage linearity over a wide dB range.
Description
RELATED APPLICATIONS

This application is related to co-pending U.S. patent application Ser. No. 09/163,892 entitled "VARIABLE GAIN AMPLIFIER WITH HIGH LINEARITY AND LOW NOISE", filed Sep. 30, 1998, and assigned to the assignee of the present invention, the teachings of which are hereby incorporated by reference.

1. Technical Field

This invention relates to electronic circuits, and more particularly to an electronic variable gain amplifier having a control voltage source that provides high gain-to-control voltage linearity over a wide dB range and a gain curve that is essentially independent of temperature.

2. Background

In a radio frequency (RF) transceiver, the received signal has a high dynamic range (>80 dB). In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. While it is desirable that the VGA gain in decibels (dB) be linear with respect to a control voltage, there is a certain range of tolerable non-linearity in gain, usually specified by gain slope variation in dB/V over some gain segment. Known solutions fail to provide a VGA that has sufficient linearity in the high gain range (typically in the top 10 dB) that are all suitable for some applications (such as a code division multiple access (CDMA) transceiver).

Attempts have been made in the past to provide such linearity by using bipolar VGA's with current steering. However, current steering circuitry implemented in bipolar technology has a hyperbolic tangent characteristic instead of the desired exponential characteristic. Attempts have been made to use feedback circuits and read-only memory based look-up tables to generate appropriate control signals for such a circuit, but such approaches are relatively slow and expensive to implement.

Accordingly, the inventor has perceived that there is a need for a variable gain amplifier having a control voltage source that provides high gain-to-control voltage linearity over a wide dB range and a gain curve that is essentially independent of temperature. The present invention provides such an amplifier.

SUMMARY

The invention includes a variable gain amplifier (VGA) having a control voltage source that provides high gain-to-control voltage linearity over at least an 80 dB gain range. Further, the gain curve for the VGA is essentially independent of temperature.

In the preferred embodiment, the VGA includes a two-stage bipolar differential amplifier. Each stage is a transconductor followed by current steering. The first stage amplifier is coupled to an exponentially varying current source to change the transconductance of the stage. The second stage amplifier is coupled to an fixed current source to maintain a fixed transconductance for the stage.

Accordingly, to obtain exponential current steering, the control signal for the current steering circuitry is pre-distorted by the following equation:

I/(1+exp(-f(VCTRL /VT))=I*A*exp(VCTRL /VREF),

where A is a scaling factor, VT =kT/q, and T is temperature in Kelvin. The invention includes a fast, inexpensive control voltage source that provides such a signal.

In particular, in one aspect the invention includes a variable gain amplifier with high gain-to-control voltage linearity over a wide dB range, including: at least one differential amplifier stage having an amplifier section and a current steering section exhibiting a hyperbolic tangent characteristic; a voltage source, coupled to the current steering section of at least one differential amplifier stage, for generating a first control signal output proportional to IEXP, and a second control signal output proportional to ICONST -IEXP, with ICONST being a constant current, and IEXP being exponentially related to an input control voltage VCTRL and independent of temperature, where the first and second control signal outputs control correct steering for the current steering section so that the current steering section has an exponential characteristic. In another aspect, the invention includes such a voltage source alone.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of the preferred embodiment of the present invention.

FIG. 2 is a block diagram of an exponential control signal generation circuit in accordance with the preferred embodiment of the present invention.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of the amplifier section of one embodiment of the present invention. The amplifier section is shown as being implemented in bipolar circuitry. In the preferred embodiment, the VGA includes a two-stage bipolar differential amplifier. Each stage is a transconductor followed by current steering. The first stage amplifier 10 is coupled to an exponentially varying current source 12 to change the transconductance of the stage. More particularly, referring to the first stage amplifier 10, differential IF inputs IN+, IN- are respectively coupled to the bases of transistors Q1 and Q2. Transistors Q1 and Q2, along with exponentially varying current source 12, form a first differential amplifier. The differential amplifier section of the second stage 14 is similar in construction, but uses a fixed current source to maintain a fixed transconductance for the stage. The second stage amplifier 14 is shown capacitively coupled to the output of the first stage amplifier 10.

For purposes of the present invention, the amplifier stages may be implemented in other configurations and have fewer or more stages. An example of another configuration for the amplifier stages is shown in the above-referenced co-pending U.S. patent application Ser. No. 09/163,892.

To improve the dynamic range of the variable gain amplifier, each the amplifier section of each stage 10, 14 is followed by corresponding current steering circuitry. More particularly, referring to the current steering circuitry of the first stage amplifier 10, paired transistors Q5-Q6 and Q7-Q8 act to steer the signal current from transistors Q1 and Q2, respectively, either to a voltage source Vcc (through transistors Q6 and Q7), or to loads coupled to outputs OUT+, OUT- (through transistors Q5 and Q8). The steering voltage signals VCTRLOUT + and VCTRLOUT - for transistors Q5, Q6, Q7, and Q8 are output by a voltage source V1. The current steering circuitry of the second stage 14 is similar in construction.

As noted above, current steering circuitry implemented in bipolar technology has a hyperbolic tangent characteristic. To obtain exponential steering, the control signal can be pre-distorted by the following equation:

I/(1+exp(-f(VCTRL /VT))=I*A*exp(VCTRL /VREF),

where A is a scaling factor, VT =kT/q, T is temperature in Kelvin, and is a function of VCTRL.

FIG. 2 is a block diagram of an exponential control signal generation circuit for generating the desired outputs for the voltage source V1. For ease of manufacture in conjunction with the amplifier stages 10, 14, this circuit is preferably implemented in bipolar circuitry. However, a comparable circuit can be implemented in other technologies, such as complimentary metal oxide semiconductor (CMOS) field effect transistors (FET). Accordingly, the embodiment shown in FIG. 2 should be taken as exemplary only.

A control voltage VCTRL is applied to a voltage-to-current converter 200 to generate a corresponding proportional current I1 equal to VCTRL /R1, where R1 is a resistance value selected empirically to scale the output current I1 for a particular application. The current I1 is applied to a translinear current multiplier/divider 202. The current multiplier/divider 202 has as additional inputs current I2 from a proportional-to-absolute-temperature (PTAT) current source, and current I3 from a constant (independent of temperature) current source. Examples of such current sources are shown in U.S. Pat. No. 5,774,013 to Groe, issued Jun. 30, 1998 and assigned to the assignee of the present invention. The current multiplier/divider 202 provides an output IOUT equal to I1 *I2 /I3, which can be expressed as VCTRL *I2 /(R1 *I3). The output current IOUT is applied to an exponential current generator 204, which has as an additional input a constant current I4. The exponential current generator 204 provides an output current IEXP that is exponentially related to IOUT and is independent of temperature. In particular, IEXP =I4 *A*exp(IOUT *R2 /VT), which can be expressed as IEXP =I4 *A*exp(VCTRL /VREF), where 1/VREF =I2 *R2 /(I3 *R1 *VT), with A being a selectable scaling factor, and R2 is a resistance value selected empirically to scale the output current IEXP for a particular application.

The output current IEXP from the exponential current generator 204 is coupled to a current replicator 206, which generates two essentially identical output currents equal to IEXP. One output of the current replicator 206 provides current flow from a voltage source VCC through a first diode 208a, thereby generating a control voltage VCRTLOUT - through a first buffer 210a that is coupled to the gates of transistors Q6 and Q7 in FIG. 1. The other output of the current replicator 206 is subtracted from a constant current ICONST by a current subtractor 212 to generate an output current ICONST -IEXP. The output current ICONST -IEXP provides current flow from a voltage source VCC through a second diode 208b, thereby generating a VCTRLOUT + control voltage through a second buffer 210b that is coupled to the gates of transistors Q5 and Q8 in FIG. 1. When the two output voltages VCTRLOUT -, VCTRLOUT + from voltage source V1 are applied to the current steering pairs of transistors, gain in dB is proportional to VCTRL.

Thus, the output of the voltage source V1 is a function of VCTRL, and provides the desired pre-distorted control signals necessary to obtain exponential steering of the current steering circuitry. In one embodiment of invention, a VGA was fabricated having a gain-to-control voltage linearity over at least an 80 dB gain range. Further, the gain curve for this VGA was essentially independent of temperature.

A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Patent Citations
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US6542029Oct 11, 2000Apr 1, 2003Skyworks Solutions, Inc.Variable-slope variable-gain amplifier
US6583667 *Dec 20, 2001Jun 24, 2003Institute Of MicroelectronicsHigh frequency CMOS differential amplifiers with fully compensated linear-in-dB variable gain characteristic
US6600371 *Nov 21, 2001Jul 29, 2003Stmicroelectronics S.R.L.Low noise amplifier
US6711391Oct 10, 2000Mar 23, 2004Qualcomm, IncorporatedGain linearizer for variable gain amplifiers
US6798290 *Aug 27, 2002Sep 28, 2004Sequoia CommunicationsTranslinear variable gain amplifier
US6873830 *May 10, 2001Mar 29, 2005Sony CorporationBias circuit and radio communication apparatus using same
US6891423 *Mar 6, 2003May 10, 2005Telefonaktiebolaget Lm Ericsson (Publ)Quadrature switching mixer with reduced leakage
US6906592Nov 3, 2003Jun 14, 2005Qualcomm IncContinuously variable gain radio frequency driver amplifier having linear in decibel gain control characteristics
US7050335Jun 14, 2004May 23, 2006Stmicroelectronics S.A.Flash memory comprising means for checking and refreshing memory cells in the erased state
US7075368 *Apr 20, 2004Jul 11, 2006Realtek Semiconductor Corp.Linear-in-decibel variable gain amplifier
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US7091786Feb 16, 2004Aug 15, 2006Realtek Semiconductor Corp.Linear-in-decibel variable gain amplifier
US7227412 *Dec 29, 2004Jun 5, 2007Stmicroelectronics S.A.Attenuation cell with an attenuation factor control device
US7443210 *Jul 10, 2006Oct 28, 2008National Tsing Hua UniversityTransmission circuit for use in input/output interface
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Classifications
U.S. Classification330/254, 327/359, 330/256, 327/356
International ClassificationH03G7/06
Cooperative ClassificationH03G7/06
European ClassificationH03G7/06
Legal Events
DateCodeEventDescription
Mar 26, 2012FPAYFee payment
Year of fee payment: 12
Apr 7, 2008REMIMaintenance fee reminder mailed
Mar 26, 2008FPAYFee payment
Year of fee payment: 8
Mar 26, 2004FPAYFee payment
Year of fee payment: 4
Oct 6, 2003ASAssignment
Owner name: ALPHA INDUSTRIES, INC., MASSACHUSETTS
Free format text: RELEASE AND RECONVEYANCE/SECURITY INTEREST;ASSIGNOR:CONEXANT SYSTEMS, INC.;REEL/FRAME:014580/0880
Effective date: 20030307
Owner name: ALPHA INDUSTRIES, INC. 20 SYLVAN ROADWOBURN, MASSA
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Sep 16, 2002ASAssignment
Owner name: CONEXANT SYSTEMS, INC., CALIFORNIA
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Effective date: 20020625
Owner name: CONEXANT SYSTEMS, INC. 4311 JAMBOREE ROADNEWPORT B
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